I've made a bench power supply that puts out 12Vdc, 5Vdc and 3.3Vdc. How can I now make it variable ?

Question

I've tried a potentiometer but I can not get it to work. I'm not sure if I'm hooking it up correctly.

Answer 1

I gotta go with Eugene on this one, best way is to add a variable voltage regulator from one of the outputs and you can then change the output voltage. If you just use a pot, then you will have to keep checking the output voltage (under load) to make sure you haven't loaded your power supply.
The circutry isn't too difficulty to build, you could easily make one fit on a 2" square brassboard (if you can solder).

And you mentioned you weren't sure if you hooked the pot up correctly, just go from center contact to either side contact (that works like 99.9%) if you go to both sides, you turn your pot into a fixed resistor. Also, you might want to make sure it has a linear taper (The resistance changes the same amount per revolution), to make 'dialing in' your voltages easier.

Answer 2

To make it work correctly, just a potentiometer won't work well beacuse the output will be load dependent. You'll need another regulator (with feedback of course) if you want to keep the other three outputs with a variable reference going to it.

Answer 3

You can use a dimmer switch, which is nothing more than a rheostat.